Referring to FIG. 1, there is shown a quadrature encoder system in which a resolver or like position sensor produces two sinusoidal signals in quadrature (90 spatial degrees out-of-phase). The frequency of the signals is proportional to the rate of change of position. The direction of the change in position may be determined from the lead-lag relationship of the two signals. The two signals are processed in separate channels (the SIN channel and the COS channel) and applied to the quadrature encoder which generates a speed and/or position signal. An important advantage of the quadrature encoder is that it can determine the direction of change in position. There are numerous techniques for encoding the quadrature signals including tracking loops, arc tangent encoding and phase encoding. Each of these techniques suffers from a sensitivity to amplitude, phase and bias errors in the quadrature signals.
A typical application of a quadrature encoder system is in the control of machine elements. Properly encoded position information can enable determination whether a shaft or other machine element is operating properly. In addition, knowledge of the position of the shaft or other machine element enables closed loop (feedback) control of the machine element.
A resolver is a transducer device which monitors the position of a rotatable shaft. A linear displacement detector is a transducer which monitors the linear displacement of a machine element or the like. Quadrature encoding systems apply equally to both.
Referring to FIG. 2, there is shown a schematic diagram of one type of resolver. At least one stator winding is fixed relative to a rotor carrying two windings that are at right angles. If a voltage signal A.multidot.sin (.omega.t) is applied across terminals S1 and S2 of the stator windings, the voltage signal across terminals R1 and R2 of the first rotor winding is A.multidot.sin(.omega.t) sin.theta. and the signal across terminals R3 and R4 of the second rotor winding is A.multidot.sin (.omega.t) cos.theta.. Here .theta. is the mechanical displacement of the rotatable shaft and Asin(.omega.t) is the stator excitation voltage.
In all quadrature encoding systems, the quadrature signals should be free of scale factor, phase and bias errors at the input to the quadrature encoder. Amplitude errors (dks and dkc) and bias errors (So and Co) are those errors that are the result of fundamental transducer gain imbalance and signal processing gain differences between channels. Spatial phase error (.phi.) can be introduced by the basic construction of the transducer (first and second rotor windings in the resolver of FIG. 2 that are not perfectly orthogonal, for example). This error can also be caused by differential impedance characteristics of the two channels. If these errors exist, then the resulting encoded angular or linear displacement information will be in error.
The prior art techniques for minimizing the errors in the quadrature signals comprise: 1) matched channel gains to minimize gain error, 2) low offset amplifiers to reduce bias, 3) ac coupling to reduce bias, 4) system calibration, and 5) cross channel coupling to reduce spatial quadrature errors. The present implementation of these techniques tends to increase costs, introduce other errors, and in all cases assume that the parameters do not change over time.